The principal reason for following the WeakEvent pattern is when the event source has an object lifetime that is potentially independent of the event listeners. Using the central event dispatching of a WeakEventManager allows the listener's handlers to be garbage collected even if the source object persists. By contrast, a normal event hookup using += causes the potentially disconnected source to hold a reference to the listeners, thus keeping the receiver from being collected in a timely fashion.

One common situation where the lifetime relationships between sources and listeners suggests the use of this pattern is the handling of update events coming from sources for data bindings.

The pattern can also be used for callbacks as well as for true events.

Notes to Inheritors:
Classes that derive from WeakEventManager class should do the following:

Override StartListening to cast the source to the type that owns the event, and connect the handler on the source to the event being managed.

Override StopListening to cast the source to the type that owns the event, and disconnect the handler on the source to the event being managed.

Implement the handler, which should call DeliverEvent, so that the managed event is forwarded to its weak event pattern listeners.

Provide a CurrentManager property that returns the specific manager type being implemented. The get accessor for CurrentManager should call GetCurrentManager to make sure that there is not already an initialized instance. If so, that instance is returned, properly typed. If there is no initialized instance, the get accessor should call SetCurrentManager to initialize one.